Process for producing allyl acetate

ABSTRACT

A process for producing allyl acetate is disclosed. The process comprises reacting a feed comprising propylene, acetic acid, oxygen, and carbon dioxide in the presence of a supported palladium catalyst. The feed comprises from 2 to 6 mole percent carbon dioxide, which improves the selectivity to allyl acetate.

FIELD OF THE INVENTION

This invention relates to a process for producing allyl acetate frompropylene, acetic acid, and oxygen in the presence of a supportedpalladium catalyst and carbon dioxide.

BACKGROUND OF THE INVENTION

Oxidation of propylene in the presence of acetic acid catalyzed by apalladium catalyst to produce allyl acetate is known. The processincludes a reaction (acetoxylation) of propylene with oxygen and aceticacid to form a mixture comprising allyl acetate, propylene, oxygen,acetic acid, water, carbon dioxide, and possibly other inert gases. Thereaction mixture is typically separated into a gas stream comprisingpropylene, oxygen, acetic acid, water, and carbon dioxide, and a liquidstream comprising allyl acetate, acetic acid, and water. Allyl acetatecan be separated from the liquid stream. At least a portion of theacetic acid and water separated from the liquid stream is recycled tothe acetoxylation reaction.

The gas stream is also generally recycled to the acetoxylation reaction(U.S. Pat. Nos. 3,970,713 and 4,010,198). U.S. Pat. No. 4,010,198discloses that the feed entering the reactor contains significantconcentration of carbon dioxide. In Example 1 of U.S. Pat. No.4,010,198, the feed contains 10 volume percent carbon dioxide. InExample 4, it contains 65 volume percent carbon dioxide. A higher carbondioxide concentration in the feed to the acetoxylation reactiondecreases the productivity of the process. In addition, the recycle gasneeds to be pressurized before it can enter the reactor because theacetoxylation is performed at a higher pressure. Consequently, a highercarbon dioxide concentration in the recycle gas requires higher energyconsumption for compressing the recycle gas.

SUMMARY OF THE INVENTION

This invention is a process for producing allyl acetate. The processcomprises reacting a feed comprising propylene, acetic acid, oxygen, andcarbon dioxide in the presence of a supported palladium catalyst. Thefeed comprises from 2 to 6 mole percent (mol %) carbon dioxide.

DETAILED DESCRIPTION OF THE INVENTION

The process comprises reacting a feed comprising propylene, acetic acid,oxygen, and carbon dioxide in the presence of a supported palladiumcatalyst, wherein the feed comprises from 2 to 6 mol % carbon dioxide.

The process uses a supported palladium catalyst. The amount of palladiumis 0.1 to 5.0 weight percent (wt %), preferably 0.3 to 1.5 wt % of thesupported catalyst.

In addition to palladium, the catalyst may comprise a Group 11 element,i.e., gold, copper, silver, and mixtures thereof. The content of gold,copper, or silver may be in the range of 0 to 5.0 wt %, preferably inthe range of 0.02 to 1.0 wt % of the supported catalyst.

The catalyst may additionally comprise an activator. An activator is analkali or alkaline earth metal compound, examples of which arehydroxides, acetates, nitrates, carbonates, and bicarbonates ofpotassium, sodium, cesium, magnesium, barium, and the like. Potassiumand cesium salts are preferred activators. The activator content may bein the range of 0 to 15 wt %, preferably 1.5 to 10 wt % of the supportedcatalyst.

The supported palladium catalyst comprises a carrier. Suitable carriersinclude alumina, silica, titania, carbon, and like, and mixturesthereof. Preferably, the carrier has a surface area of at least 1 m²/gand a pore volume of 0.1 to 1.5 m L/g.

The catalyst may be prepared by many techniques. Examples of thesetechniques are disclosed in U.S. Pat. Nos. 3,925,452, 5,011,980,6,303,536, and U.S. Pat. Appl. Pub. Nos. 2006/0167307 and 2006/0247462.

In preparing the catalyst, the carrier can be simultaneously orsuccessively impregnated with a palladium compound, optionally a Group11 metal salt, and optionally an activator. Preferably, the impregnationis performed in aqueous solutions.

Suitable palladium compounds include palladium chloride, sodiumchloropalladate, palladium nitrate, palladium sulfate, the like, andmixtures thereof. Suitable Group 11 metal salts include chlorides,nitrates, sulfates. Examples are tetrachloroauric acid, sodiumtetrachloroaurate, copper chloride, copper nitrate, copper sulfate,silver nitrate, the like, and mixtures thereof. Suitable activatorsinclude hydroxides, carbonates, bicarbonates, metasilicates of alkaliand alkaline earth metals, the like, and mixtures thereof.

One method to impregnate the carrier involves contacting the carrierwith an aqueous solution containing both a palladium compound and aGroup 11 metal salt. In another method, the carrier is contacted with apalladium compound and a Group 11 metal salt in separate steps.

An alkali metal, alkaline earth metal, or ammonium compound isoptionally contacted with the carrier during or after the carrier isimpregnated with the palladium compound and optionally the Group 11metal salt. These compounds help the palladium compound and the Group 11metal salts, if used, to bind to the carrier. Suitable alkali metal,alkaline earth metal, or ammonium compounds include their hydroxides,carbonates, bicarbonates, metasilicates, and the like, and mixturesthereof. The impregnated carrier is optionally washed with water or anaqueous solution.

The impregnated carrier is usually calcined (heated at an elevatedtemperature) in a non-reducing atmosphere. Preferably, the calcinationof the impregnated carrier is carried out at a temperature in the rangeof about 100 to about 600° C., more preferably, in the range of 250 to500° C. Suitable non-reducing gases for the calcination include helium,nitrogen, argon, oxygen, air, carbon dioxide, the like, and mixturesthereof. Preferably, the calcination is carried out in an atmosphere ofnitrogen, oxygen, air, or mixtures thereof.

Following the calcination, the resulting material is normally reduced toconvert at least a portion of the palladium and the Group 11 metal, ifused, to the corresponding elements with zero valence. The reduction isperformed by contacting it with a reducing agent. Suitable reducingagents include hydrogen, carbon monoxide, olefins, aldehydes, alcohols,hydrazine, the like, and mixtures thereof. Temperatures employed for thereduction are in the range of 20 to 700° C.

Hydrogen gas is a preferred reducing agent. Generally, a mixed gascontaining hydrogen and another gas such as argon, helium, nitrogen, orthe like, is used. Preferably, the reduction temperature is in the rangeof 300 to 700° C. Most preferably, the reduction temperature is in therange of 450 to 550° C.

The feed to the reaction comprises propylene, acetic acid, oxygen, andcarbon dioxide. The feed preferably comprise an inert gas. Examples ofsuitable inert gases include propane, nitrogen, helium, argon, the like,and mixtures thereof. The amount of inert gas in the feed is preferablyin the range of 10 to 55 mol %, more preferably from 15 to 25 mol %.

The reaction mixture comprises allyl acetate, propylene, oxygen, aceticacid, water, and carbon dioxide. Typically, the reaction mixture ispartially condensed to form a liquid stream which can be separated froma remaining gas stream. The liquid stream typically contains 5 to 20 wt% allyl acetate, 20 to 40 wt % acetic acid, and about 40 to 60 wt %water. Depending upon the concentrations of components, the liquidstream may be separated into an organic stream comprising allyl acetate,and an aqueous stream comprising water and acetic acid. Preferably, theaqueous stream or a portion of it (called “recycle liquid”) is recycledto the reaction.

The gas stream can be compressed and recycled. There are varioustechniques to recycle the gas stream. In one example, a recycle gas ispassed through an evaporator containing acetic acid and water so thatthe recycle gas is charged with the requisite quantity of acetic acidand water from the evaporator before it enters the reactor.

Depending upon the temperature and pressure at which the gas stream isseparated from the liquid stream, the gas stream may contain certainquantities of condensable products such as water, allyl acetate, andacetic acid. Such a gas stream can be directly recycled to theacetoxylation reaction.

The feed to the acetoxylation reaction usually includes not only thecomponents introduced to the process including propylene, oxygen, andacetic acid, but also any recycle stream (i.e., the recycle gas and therecycle liquid) from the process. The content of propylene in the feedis generally between 20 to 80 mol %, preferably 40 to 70 mol %. Apropylene content of greater than 50 mol % is particularly desirable.Commercially available products such as polymer grade propylene andchemical grade propylene are suitable sources of propylene. Preferably,the source of propylene has a purity of at least 90 mol %.

The feed comprises typically 8 to 20 mol %, preferably from 10 to 18 mol%, acetic acid.

The feed comprises typically 2 to 8 mol %, preferably 3 to 6 mol %oxygen. The oxygen source in the present invention is not limited, andmay be supplied in the form of a mixture with a gas such as nitrogen orcarbon dioxide. Air may be used. Preferably the oxygen source has apurity of at least 90 mol %, more preferably at least 95 mol %. Theallowed oxygen concentration in the feed is determined by theflammability limit. The flammability limit depends on temperature,pressure, and composition. It can be shifted by additional components,such as acetic acid, water, nitrogen, carbon dioxide, and argon.

The feed comprises from 2 to 6 mol %, more preferably from 3 to 6 mol %,most preferably from 4 to 6 mol % carbon dioxide. Carbon dioxide notonly serves as a diluent in the feed, but also improves the selectivityto allyl acetate and suppresses the formation of carbon dioxide (seeTables 1 and 2).

The concentration of carbon dioxide in the feed may be controlled byseveral techniques. For example, carbon dioxide can be removed from therecycle gas by adsorption, scrubbing, or a purge so that the carbondioxide concentration in the feed remains more or less constant.

The feed may comprise water. The amount of water present in the feed ispreferably from 0 to 5 mol %, more preferably from 1 to 4 mol %.

The feed is a gas under the reaction conditions. Accordingly, thequantities of acetic acid and water in the feed are adjusted so that thefeed is gaseous under the temperature and pressure selected for thereaction. The reaction is generally performed at a temperature in therange of 100 to 250° C., preferably 120 to 200° C. Generally, thereaction pressure is in the range of 15 to 450 psig, preferably in therange of 30 to 150 psig.

The reaction may be performed in a fixed bed reactor or a fluidized bedreactor, or the like. A fixed bed reactor is preferred. In one example,a multitubular fixed bed reactor is used. Typically the tube diameter isfrom 1″ to 4″ (U.S. Pat. No. 3,970,713).

The feed preferably passes through the catalyst at a space velocity ofin the range of 10 to 15,000 h⁻¹, more preferably in the range of 300 to8,000 h⁻¹.

Propylene conversion is generally 3 to 15%, and that of acetic acid 9 to45%. Oxygen conversion can be up to 90%.

EXAMPLE 1 Catalyst A

A catalyst precursor, Pd/Au/alumina, is prepared by following theprocedure disclosed in Example 1 of U.S. Pat. No. 6,022,823. After thematerial (43 g) is reduced, it is mixed with an aqueous cesium acetatesolution (25 wt %, 16.5 g). The sample is then dried overnight at 120°C. in air. The calculated composition of the obtained catalyst (CatalystA) is: 1.1 wt % Pd, 0.5 wt % Au, and 6.0 wt % Cs.

EXAMPLE 2 Catalyst B

NaAuCl₄.2H₂O (0.57 g), Na₄PdCl₄.3H₂O (1.71 g), and water (16 g) areadded to a 150-mL beaker equipped with a stirrer bar. NaHCO₃ (1.68 g) isadded in the beaker in three equal portions. Carbon dioxide is slowlyreleased. The solution formed is added dropwise to titania extrudates(27 m²/g, crushed to 14×30 mesh) while they are tumbling in a rotatingbowl.

While the impregnated solid is rotating in the bowl, it is heated with ahot air gun until it is free flowing. The dried solid is further driedin an oven at 80° C. for 12 h.

The above solid (50 g) is rinsed with hot (80° C.) deionized water (4 L)to remove chloride from the solid. The washed solid is dried at 80° C.for 6 h, further dried at 125° C. for 2 h, then cooled to roomtemperature. It is then heated under an air flow at a rate of 50standard liters per hour from room temperature to 120° C. at a rate of20° C./min, held at 120° C. for 10 min, heated from 120 to 220° C. at arate of 1.5° C./min, held at 220° C. for 3.5 h, then cooled to roomtemperature at 40° C./min. The solid is then purged with nitrogen (50standard liters per hour) for 30 min.

The above material is treated with a gas mixture containing hydrogen andhelium in a molar ratio of 5:95 (50 standard liters per hour). Thetemperature is raised from room temperature to 220° C. at a rate of 30°C./min, held at 220° C. for 10 min, raised again to 500° C. at a rate of5° C./min, and held at 500° C. for 3.25 h. It is then cooled to roomtemperature while being purged with nitrogen (50 standard liters perhour) for 15 min. A catalyst precursor is obtained.

An aqueous cesium acetate solution is prepared by dissolving cesiumacetate (50 g) in water (150 g). Copper acetate monohydrate (0.19 g) isadded to a portion of the above aqueous cesium acetate solution (6.1 g).The obtained solution is added dropwise to a portion of the abovecatalyst precursor (19.6 g). The resulted material is dried overnight at120° C. in air. The calculated composition of the obtained catalyst(Catalyst B) is: 0.9 wt % Pd, 0.5 wt % Au, 0.3 wt % Cu, and 4.9 wt % Cs.

EXAMPLE 3 Catalyst C

Catalyst C is prepared according to the procedure of Example 2 exceptthat a solution is prepared by mixing copper acetate monohydrate (0.41g) and the CsOAc solution (6.1 g). The obtained solution is addeddropwise to a portion of the catalyst precursor prepared in Example 2(19.6 g). The product is dried overnight at 120° C. in air. Thecalculated composition of the obtained catalyst (Catalyst C) is: 1.1 wt% Pd, 0.5 wt % Au, 0.3 wt % Cu, and 6.0 wt % Cs.

EXAMPLE 4 Catalyst D

Catalyst D is prepared according to the procedure of Example 2 exceptthat a solution is prepared by mixing copper acetate monohydrate (0.06g) and a potassium acetate solution (12.8 wt %, 7.7 g). The solution isadded to a portion of the catalyst precursor prepared in Example 2 (20g), which is then dried overnight at 120° C. The calculated compositionof Catalyst D is: 1.1 wt % Pd, 0.5 wt % Au, 0.1 wt % Cu, and 1.9 wt % K.

EXAMPLE 5 Catalyst E

Catalyst E is prepared according to the procedure of Example 2 exceptthat a solution is prepared by mixing copper acetate monohydrate (0.19g) and a potassium acetate solution (12.8 wt %, 7.4 g). The solution isadded to a portion of the catalyst precursor prepared in Example 2 (20g), which is then dried overnight at 120° C. The calculated compositionof Catalyst E is: 1.0 wt % Pd, 0.5 wt % Au, 0.3 wt % Cu, and 1.8 wt % K.

EXAMPLE 6 Catalyst F

A solution is prepared by dissolving Na₂PdCl₄.3H₂O (1.4 g), NaHCO₃ (1.4g) in water (10 g). The solution is used to impregnate alpha alumina (5/16″ pellets, surface area=4 m²/g, 35 g) according to procedure ofExample 2. A catalyst precursor (Precursor G) is produced.

A second solution is prepared by mixing copper acetate monohydrate (0.19g) and a potassium acetate solution (12.8 wt %, 7.5 g). The solution isadded to 20 g of Precursor G prepared above, which is then driedovernight at 120° C. The calculated composition of catalyst obtained(Catalyst F) is: 1.2 wt % Pd, 0.3 wt % Cu, and 1.7 wt % K.

EXAMPLE 7 Acetoxylation with Catalyst A

A stainless steel reactor (0.97″ ID equipped with a 0.25″ thermowell atthe center) is packed with a mixture of Catalyst A (10 mL) and glassbeads (1 mm diameter, 30 mL). The reactor is heated by a sand bath to140° C. A feed containing 58 mol % propylene, 3 mol % oxygen, 15 mol %acetic acid, carbon dioxide (concentrations shown in Table 1), andnitrogen (balance) is fed to the reactor at a flow rate of 22 standardliters per hour. The reactor pressure is controlled at 80 psig. Thereaction mixture is cooled to room temperature and separated by avapor/liquid separator to a liquid stream and a vapor stream. Thereaction continues for 500 h. The liquid stream and the vapor stream areanalyzed by gas chromatography (GC). The results are listed in Table 1.

Table 1 shows that 2 mol % carbon dioxide in the feed improves theselectivity to allyl acetate and reduces the selectivity to carbondioxide. At about 4 mol % carbon dioxide, the selectivity to carbondioxide is reduced to an undetectable level.

EXAMPLES 8-12 Acetoxylation with Catalysts B, C, D, E, and F

The procedure of Example 7 is repeated except that Catalysts B, C, D, E,and F are used instead of Catalyst A. The results are listed in Table 2.Table 2 shows that selectivities to allyl acetate are improved forvarious supported palladium catalysts by including 4.0 mol % carbondioxide in the feed.

TABLE 1 Feed Selectivity Selectivity Selectivity Carbon Dioxide toCarbon Dioxide to AAc* to ADAc* STY* Test (mol %) (%) (%) (%) (g AAc/Lcat/h) Comparative 7.1 0 3.6 91.0 1.3 263 Comparative 7.2 1.0 4.0 90.71.1 254 7.3 2.0 2.6 92.3 1.1 263 7.4 3.0 1.2 92.9 1.0 251 7.5 4.0 0 94.41.1 251 7.6 5.0 0 94.2 1.1 245 7.7 5.7 0 94.3 1.1 242 *AAc = allylacetate; ADAc = allyl diacetate; STY = space time yield, expressed asgrams of allyl acetate produced per liter of catalyst per hour.

TABLE 2 Feed Selectivity Selectivity Selectivity Carbon Dioxide toCarbon Dioxide to AAc* to ADAc* STY* Test Catalyst (mol %) (%) (%) (%)(g AAc/L cat/h) Comparative 8.1 B 0 1.2 94.0 1.7 330  8.2 B 4.0 0 95.21.6 318 Comparative 9.1 C 0 1.5 97.8 0.2 210  9.2 C 4.0 0 98.2 0.2 200Comparative 10.1 D 0 0.7 97.8 0.2 206 10.2 D 4.0 0 98.4 0.2 203Comparative 11.1 E 0 1.1 94.2 1.4 324 11.2 E 4.0 0 95.4 1.3 321Comparative 12.1 F 0 0.6 96.8 0.6 278 12.2 F 4.0 0 97.5 0.6 263 *AAc =allyl acetate; ADAc = allyl diacetate; STY = space time yield, expressedas grams of allyl acetate produced per liter of catalyst per hour.

1. A process for producing allyl acetate, comprising reacting a feedcomprising propylene, acetic acid, oxygen, and carbon dioxide in thepresence of a supported palladium catalyst, wherein the feed comprisesfrom 2 to 6 mol % carbon dioxide, further wherein the concentration ofthe carbon dioxide in the feed is controlled at a constantconcentration.
 2. The process of claim 1 wherein the feed comprises from3 to 6 mol % carbon dioxide.
 3. The process of claim 1 wherein the feedcomprises from 4 to 6 mol % carbon dioxide.
 4. The process of claim 1wherein the feed comprises 1 to 4 mol % water.
 5. The process of claim 1wherein the feed comprises greater than 50 mol % propylene.
 6. Theprocess of claim 1 wherein the feed comprises an inert gas selected fromthe group consisting of propane, argon, nitrogen, and mixtures thereof.7. The process of claim 6 wherein the feed comprises 10 to 55 mol %inert gas.
 8. The process of claim 1 wherein the feed comprises 8 to 20mol % acetic acid.
 9. The process of claim 1 wherein the feed comprisesfrom 2 to 8 mol % of oxygen.
 10. The process of claim 1 wherein theconcentration of the carbon dioxide is controlled by absorption.
 11. Theprocess of claim 1 wherein the concentration of the carbon dioxide iscontrolled by scrubbing.
 12. The process of claim 1 wherein theconcentration of the carbon dioxide is controlled by purge.
 13. Theprocess of claim 1 wherein propylene has a purity of at least 90 mol %.14. A process for producing allyl acetate, comprising reacting a feedcomprising propylene, acetic acid, oxygen, and carbon dioxide in thepresence of a supported palladium catalyst, wherein the feed comprisesfrom 2 to 6 mol % carbon dioxide, further wherein the concentration ofthe carbon dioxide in the feed is controlled at a constantconcentration, wherein the selectivity to allyl acetate is improved whencompared to a process wherein less than 2 mol % carbon dioxide ispresented in the feed.
 15. A process for producing allyl acetate,comprising reacting a feed comprising propylene, acetic acid, oxygen,and carbon dioxide in the presence of a supported palladium catalyst,wherein the feed comprises from 2 to 6 mol % carbon dioxide, furtherwherein the concentration of the carbon dioxide in the feed iscontrolled at a constant concentration, wherein the formation of carbondioxide is suppressed when compared to a process wherein less than 2 mol% carbon dioxide is present in the feed.
 16. The process of claim 1wherein the palladium catalyst further comprises gold, copper, silver,or mixtures thereof.
 17. The process of claim 1 wherein the palladiumcatalyst further comprises an activator.
 18. The process of claim 17wherein the activator is selected from the group consisting of apotassium salt, a cesium salt, and combinations thereof.
 19. The processof claim 1 wherein the palladium catalyst further comprises a carrier.20. The process of claim 19 wherein the carrier is alumina, silica,titania, carbon, or a mixture thereof.